As commentators quickly recognized, there’s just cause for celebration in this week’s Supreme Court decision in Riley v. California, requiring a warrant to search an arrestee’s cell phone. The result was by no means a foregone conclusion because the Court had long held that police may search an arrestee and everything in his possession without a warrant and without any reasonable basis for suspicion. Of course, cell phones cannot threaten an officer’s safety, they typically contain intensely personal information, and once they are seized, the arrestee cannot tamper with any evidence they may hold. But all this is true of the arrestee’s wallet, handbag, diary and personal papers, all of which can be inspected under the “search incident to arrest” authority conferred by prior precedent. The Court had reasoned that an arrestee’s diminished expectations of privacy and the difficulty of distinguishing between different kinds of material that police encounter require a per se authority to examine everything they might find on an arrestee’s person.

Riley now limits this categorical authority to hard-copy papers and other tangibles. Police can flip open an old-style cell phone or a computer laptop to make sure there is no razor blade hidden inside, but they cannot search its digital data without a warrant. In recognizing that digital data “differ in both a quantitative and a qualitative sense,” because they hold so much personal information, Riley has broad implications. Justice Sotomayor wrote only for herself when she made a similar point two years ago in the GPS tracking case (United States v. Jones), but in Riley, Chief Justice Roberts, writing for eight members of the Court, now quotes approvingly from her analysis. A wide range of issues involving computer searches, electronic surveillance and data mining will now have to be revisited, with heightened attention to the strong Fourth Amendment privacy interests at stake.

Six points in particular are worth noting:

Privacy gets renewed prominence as a constitutional value. Chief Justice Roberts closes his opinion with a peroration to the Fourth Amendment, recalling that abusive searches “were one of the driving forces behind the Revolution itself” and that shielding personal information from the government was a “protection for which the Framers fought.” Contrary to the prevalent view that technology has rendered privacy obsolete, the Court sees recent developments as a reason to strengthen Fourth Amendment safeguards.

The “nothing to hide” argument can be buried forever. A persistent theme in public defenses of government surveillance has been the specious argument that good citizens have nothing to fear because they should have nothing to hide. The Roberts opinion explains vividly and in detail why law abiding Americans have legitimate needs to shelter the countless personal details that computer data and internet histories can reveal.

Legislative compromises are probably unacceptable. Justice Alito wrote separately to argue, as he did in Jones, that legislatures are well positioned to fashion appropriate limits on cell phone searches by balancing the relevant privacy and law enforcement interests. He reasons that the Court should defer to lines that might be drawn by democratic deliberation. As I discuss in detail in my recent book (More Essential than Ever: The Fourth Amendment in the Twenty-First Century), this effort to rely on the political process is profoundly misguided. The Fourth Amendment shelters the thoughts and associations of minorities, non-conformists and dissenters; its concerns cannot be met by porous safeguards that mainstream voters may consider “reasonable.” This point is crucial in setting the boundaries of GPS tracking and electronic surveillance; it applies with even greater force in the context of search incident to arrest. Law enforcement interests, through the Department of Justice, have outsized influence (and literally a veto) in the legislative process. And citizens do not share on anything near equal terms the risk of being arrested for crime, or even the risk of facing custodial arrest for a traffic violation. An important bit of good news not widely noticed is that no other member of the Court was willing to endorse Justice Alito’s democratic-process approach. The Court’s opinion does not mention the Alito option, and it may be that some Justices remain agnostic on the subject. But it would have been easy enough for the Chief Justice to advert to it in some noncommittal way; that he chose not to do so, even at the sacrifice of the fully unanimous opinion he no doubt sought, is a potentially telling detail. And more concretely, the opinion’s strong account of the values that lie at the heart of the Fourth Amendment affords some reason for optimism that when the time comes, the Court will understand the need to reaffirm the case-specific judicial checking function, even when legislation like the Foreign Intelligence Surveillance Act chooses to dilute it.

Executive Branch safeguards fare even worse. In answer to a government argument that unjustified spill-over in cellphone searches could be cabined by executive regulations, the Court dismissively admonished that “the Founders did not fight a revolution to gain the right to government agency protocols.”

Warrantless section 702 surveillance of non-US persons abroad is now especially suspect. Just as a cell phone search will inevitably stumble on remotely stored information NOT in the possession of the arrestee (and therefore not subject to search under search-incident-to-arrest power), section 702 surveillance of foreign targets inevitably sweeps up conversations with US persons that are not in themselves subject to warrantless seizure. The statute seeks to address this concern by requiring the Executive Branch to promulgate regulations that limit the retention and use of US-person material. But apart from the fact that such regulations are secret, and that they apparently do not wholly prohibit the retention and use of such material, Riley’s admonition could be decisive here: “the Founders did not fight a revolution to gain the right to government agency protocols.” [For more on this point, see Wednesday’s post by our Just Security colleague Jennifer Granick.] Riley clearly can be read to hold that a case-specific judicial assessment of probable cause is constitutionally required where the Fourth Amendment rights of US citizens are implicated.

The third party doctrine is badly shaken but still intact. One of the principal arguments supporting the government’s position in Riley was the claim that under the third party doctrine, most of the information held in cell phones carries no reasonable expectation of privacy because it has been voluntarily turned over to a third party intermediary. Riley reinforces the counterargument that such information can be intensely personal and that we do not expect it to be broadcast to the whole world. But in a Fourth Amendment framework, the crucial point in Riley (a point stressed in footnote 1 of the opinion) was the way the police got the information – by manipulating the arrestee’s phone. Even before Katz held that oral communication is protected by the Fourth Amendment, the law was clear that government could not use conversations overheard by trespassing on private property. Such conversations, even if unprotected in themselves, were the tainted fruit of an unwarranted search. Bank records cannot be used if found on a desk during an illegal search of the home, but police can get them without a warrant or probable cause, simply by going directly to the bank. At bottom, Riley shows the Court’s full appreciation of the threat posed by unrestricted government access to digital files, and this may ultimately prove to be its most important legacy. But the logic of the third-party doctrine still will have to be tackled head-on in situations where police get information directly from an intermediary like an internet service provider or a cloud computing service.